Medical treatments of disorders caused by abnormal organ function typically employ pharmaceutical agents designed for either compensating for such abnormal organ function or treating the dysfunctional organ tissue. However, in some cases, pharmaceutical therapy cannot be instated since organ function is oftentimes complex and/or not completely understood.
In such cases, the only viable alternative is surgical replacement of the non-functional organ, which is now widely used for treatment of liver and kidney failure, both acute and chronic, as well as for cancer and certain inborn abnormalities. However, the need for donor organs far exceeds the supply. Organ shortage has resulted in new surgical techniques, such as splitting adult organs for transplant. Despite fairly good results, such techniques still suffer from a lack of donor tissue.
The lack of viable donor tissue has led to the emergence of stem cell replacement therapy, which relies on stem cell plasticity i.e., the ability to give rise to cell types in a new location that are not normally present in the organ in which the stem cells are located.
Stem cells are generally classified according to their origin, essentially adult, embryonic or neonatal origin. Embryonic stem cells deriving from the inner cell mass of the blastocyst are pluripotential, bring capable of giving rise to cells found in all three germ layers. Despite long held belief adult stem cells are not as lineage restricted as previously thought. In particular, haematopoietic and neural stem cells appear to be the most versatile at cutting across lineage boundaries. For example, recent reports suggest that hematopoietic stem cells (HSCs) of human origin have a hepatic potential. Studies of liver or bone marrow transplantation from sex mismatched donors, identified bone marrow-derived hepatocytes in recipients [Alison (2000) Nature 406:257; Theise (2000) Hepatology 32:11-16; Korbling (2002) N Engl J Med 346:738-746.]. Murine and rat HSCs were also found to migrate to irradiated or injured adult livers, and to differentiate into hepatic cells [Petersen (1999) Science 284:1168-1170; Theise (2000) Hepatology 31:235-240; Lagasse (2000) Nat Med 6:1229-1234]. Furthermore, single murine hematopoietic stem cell transplantation has resulted in detection of HSC-derived cells in the liver of irradiated recipients with a low percentage of transplanted cells exhibiting immunohistochemical and morphologic properties of hepatic epithelial cells [Krause (2001) Cell 105:369-377].
The mechanisms that guide circulating hematopoietic stem cells are clinically significant because the success of stem cell transplantation depends on efficient targeting (also referred to as homing) of grafted cells to the recipient target tissue [Mazo and von Adrian (1999) Journal of leukocyte Biology 66, 25-32]. It is due to this homing of transplanted cells that bone marrow transplantations do not require invasive surgery, as in the case with the transplantation of any other organ, but rather can be effected by simple intravenous infusion.
Homing of HSCs can be defined as the set of molecular interactions that allows circulating HSCs to recognize, adhere to, and migrate across bone marrow endothelial cells resulting in the accumulation of HSCs in the unique hematopoiesis-promoting microenvironment of the bone marrow. Homing of progenitor cells can be conceived as a multi-step phenomenon [Voermans (2001) J. Hematother. Stem Cell Res. 10:725-738, Lapidot (2002) Leukemia 16:1992-2003]. HSCs arriving to the bone marrow must first interact with the luminal surface of the bone marrow endothelium. This interaction must occur within seconds after the HSCs have entered the bone marrow microvasculature and provide sufficient mechanical strength to permit the adherent cell to withstand the shear force exerted by the flowing blood. Adherent HSCs must then pass through the endothelial layer to enter the hematopoietic compartment. After extravasation, HSCs encounter specialized stromal cells whose juxtaposition supports maintenance of the immature pool by self-renewal process in addition to lineage-specific HSCs differentiation, proliferation and maturation, a process that involves stroma-derived cytokines and other growth signals.
Only a limited number of factors involved in stem cells homing are known to date; these include, the ligand for c-kit, stem cell factor, which has been shown to play a central role in adherence of HSCs to the stroma; and integrin interactions (e.g., β1-Intergrins), which were shown to be crucial to the migration of HSCs to the foetal liver [Zanjani (1999) Blood 94:2515-2522]. One important molecular interaction which is considered central to HSC homing is that of chemokine stromal derived factor (SDF-1) and its cognate receptor, CXCR4.
SDF-1 is the only known powerful chemoattractant of hematopoietic stem cells of both human [Aiuti (1997) J. Exp. Med. 185:111-120] and murine origin [Wright (2002) J. Exp. Med. 195:1145-1154] known to date. SDF-1 is widely expressed in many tissues during development [McGrath (1999) Dev. Biol. 213:442-456] and adulthood [Nagasawa (1994) Proc Natl Acad Sci USA 91:2305-2309; Imai (1999) Br J Haematol 106:905-911; Pablos (1999) Am J Pathol 155:1577-1586], such as for example the liver [Shirozu (1995) Genomics 28:495-500; Nagasawa (1996) Nature 382:635-638; Goddard (2001) Transplantation 72:1957-1967]. Previously, the present inventors were able to show that bone marrow homing and repopulation by sorted human CD34+/CD38−/low stem cells transplanted into the tail vein of irradiated immune deficient NOD/SCID and NOD/SCID/B2m null mice, are dependent on SDF-1/CXCR4 interactions [Peled (1999) Science 283:845-848; Kollet (2001) Blood 97:3283-3291].
More recently, the present inventors also established a role for these interactions in G-CSF-induced mobilization of murine and human stem cells [Petit (2002) Nat Immunol 3:687-694].
In view of the ever-expanding use of stem cell therapy, it is highly desirable to further elucidate the mechanism behind stem cell homing and target repopulation in order to improve the efficiency and success rate of cell replacement therapy.
While conceiving the present invention, the present inventors have hypothesized that stress conditions may promote stem cell homing to a target tissue. This hypothesis is strongly supported by prior art studies which illustrated the following:
(i) Stem cells were found to repopulate a damaged murine liver while such finding was not be observed in parabiotic mice [Wagers (2002) Science 297:2256-2259], suggesting that repopulation does not occur under steady state homeostatic conditions in non-irradiated or non-damaged intact livers.
(ii) Although the levels of hematopoietic stem cells that engraft the irradiated liver and develop into hepatocyte-like, albumin producing cells are very low, this process can be amplified by liver injury or viral inflammation. Thus, under strong selection conditions that exist in fumarylacetoacetate hydrolase (FAH) null mice, which have ongoing severe hepatocyte damage due to deficiency of this enzyme, there is enormous amplification of transplanted, purified murine hematopoietic stem cells that demonstrate hepatic morphology and function, along with improvement of the metabolic disorder [Lagasse (2000) Nat Med 6:1229-1234].
(iii) Liver repopulation by bone marrow (BM) cells from Bcl-2 transgenic mice transplanted into wild-type recipients, followed by repeated rounds of liver injury and regeneration induced by Fas-mediated apoptosis, represents another example of selective amplification of transplanted BM cells following differentiation into hepatocytes [Mallet (2002) Hepatology 35:799-804].
(iv) High levels of bone marrow-derived hepatocytes were reported in a liver trasplant recipient in whom the transplanted liver became infected with hepatitis C virus [Theise (2000) Hepatology 32:11-16.].
Altogether these observations demonstrate the potential of hematopoietic stem cells to gain hepatic phenotype can be significantly amplified under stress conditions. However, the mechanisms and factors, which regulate stem cell recruitment to the damaged tissue and induce their desirable phenotype, are currently unknown.